1. Field of the Invention
The present invention relates to a band management circuit, which is used in a 1-to-multiple-connection communication network, for example, in a communication network in which 1 SLT (Subscriber Line Terminal) is connected to a plurality of ONU (Optical Network Units).
2. Description of Related Art
ATM (Asynchronous Transfer Mode)-PDS (Passive Double Star) is well known as one system for constructing a communication network user system. As documentation related to ATM-PDS, for example, there is documentation such as that cited hereinbelow.
(1) Nakajima and Takigawa xe2x80x9cTime Slot Assignment Method for ATM-PDS Systemxe2x80x9d 1993, Fall Meeting of the Institute of Electronics, Information and Communication Engineers of Japan, B-668
(2) Y. Takigawa et al xe2x80x9cATM based Passive Double Star system offering B-ISDN, N-ISDN and POTSxe2x80x9d IEEE 1993 pp. 14-18
In a fiber-optic network that uses ATM-PDS, 1 SLT is connected to a plurality of ONU by way of 1 SC (Star Coupler). An optical signal sent from the SLT is divided by the SC and transmitted to ONU. Further, optical signals sent from ONU are multiplexed by the SC and transmitted to the SLT.
In sending optical signals from ONU to an SLT, the transmission timing of the plurality of ONU must not overlap. For this reason, ONU transmission timing must be managed by the network. The management thereof is performed by a band management circuit provided inside the SLT. The SLT, in accordance with the control of the band management circuit, sends to an ONU an ATM-PDS cell that indicates transmission authorization. An ONU receives this ATM-PDS cell, determines on its own whether or not transmission authorization has been granted, and performs transmission only when transmission authorization has been granted. When an SLT receives an optical signal from an ONU that has been granted transmission authorization, the band management circuit grants subsequent transmission authorization to the ONU.
In an ATM-PDS system, there is no need for each ONU to have the same transmission band. The band allotted to each ONU is determined by a contract between a subscriber and a communications service. That is, the frequency with which an ONU receives transmission authorization is determined for each ONU in accordance with a contract. A band management circuit separately stores the band of each ONU, and each time determines the ONU, for which transmission is to be authorized in accordance with the band thereof. This is the reason for increasing the size of a band management circuit, and for making the structure thereof more complex. The size and complexity of a band management circuit become more conspicuous the more numerous managed ONU become.
In an ATM-PDS cell, which indicates transmission authorization, there is stored data, for example, an identification number set in each ONU, for identifying an ONU to which authorization has been granted. An ONU, which is targeted for transmission authorization, is specified in accordance with the identification data thereof. As systems for storing this identification data in an ATM-PDS cell, there is a system, which stores only 1 identification data in 1 ATM-PDS cell, and a system, which stores a plurality of identification data in 1 ATM-PDS cell. It is desirable that a band management circuit be capable of handling both systems.
An object of the present invention is to provide a band management circuit, which has a small circuit size, a simple circuit structure, and is capable of handling a variety of transmission authorization systems.
For this reason, a band management circuit related to the present invention comprises means for forming a table, which stores a plurality of identification data for specifying a terminal device for which transmission is authorized; means for generating the identification data of a transmission authorization number, which is determined in accordance with a transmission band provided to the respective terminal devices; means for writing to the forming means the identification data generated by the generating means; and means for sequentially reading the identification data from the storing means in accordance with a prescribed timing.
A band management circuit related to the present invention arranges identification data in a table, and outputs same to the outside in accordance with a prescribed timing. Consequently, a band management circuit related to the present invention need not be made larger in size or more complex, even when a large number of terminal devices are being managed. In addition, because a band management circuit related to the present invention is able to readily change the cycle for outputting identification data to the outside, this band management circuit can easily handle a variety of transmission authorization systems.